Field of the Invention
The present invention relates to a brake control method for a hybrid electric vehicle. More particularly, the present invention relates to an enhanced brake control method to cope with a hydraulic hysteresis phenomenon depending on a pedal stroke when entering a control mode to generate both regenerative brake force and hydraulic brake force from a control mode to generate only the regenerative brake force by increasing a brake pedal stroke in the hybrid electric vehicle in which a regenerative brake cooperation control is performed and thereafter, re-entering a control mode to generate only the regenerative brake force.
Description of Related Art
In a hybrid electric vehicle, when a brake control for decelerating or stopping a vehicle is executed while driving, a regenerative brake is executed by a motor to charge a battery.
A total brake amount determined by a pedal stroke generated as a driver steps a brake pedal is distributed to a regenerative brake amount and a hydraulic brake amount (friction brake amount) executed by hydraulic pressure (wheel brake pressure) supplied to each wheel brake (hydraulic brake), and as a result, a cooperation control is executed.
In some existing hybrid electric vehicles, since a regenerative brake cooperation control is not performed when braking a vehicle, an effect is not high in fuel enhancement by the regenerative brake, but thereafter, a fuel enhancement rate can be increased by applying while regenerative brake cooperation control technology and the regenerative brake cooperation control is performed through a hydraulic booster in a brake control device of the hybrid electric vehicle in the related art.
That is, in order to satisfy total brake force (driver's required brake force) depending on a driver's request, the regenerative brake cooperation control to generate the hydraulic brake force (friction brake force by a wheel brake which is the hydraulic brake) with basic brake force by the hydraulic booster and simultaneously generate regenerative brake force through a regenerative brake control is performed.
In this case, when the hydraulic booster fails, an electronic stability control system (ESC) that generates auxiliary brake force by driving a motor and a pump is used.
However, the brake control device in the related art may recover some of energy lost while braking to provide a higher fuel efficiency enhancement effect than the case where the regenerative brake cooperation control is not performed, but since the hydraulic booster directly controls pressure from a master cylinder to a wheel brake, a pedal simulator for providing pedal operation feel needs to be provided, and as a result, cost increases, thereby decreasing price competitiveness.
In order to solve the problem, the prior art discloses technology that allows an ESC to perform an anti-lock braking system (ABS) function, a traction control system (TCS) function, an ESC's own function, and a regenerative brake cooperation control function in the hybrid electric vehicle.
The brake control device of the hybrid electric vehicle that performs the regenerative brake cooperation control in the prior art includes a master cylinder generating brake hydraulic pressure depending on the pedal stroke, an ESC controlling the brake hydraulic pressure supplied to the wheel brake from the master cylinder while regenerative braking, and a wheel brake connected with the master cylinder through the ESC.
Herein, the ESC includes an outlet valve capable of discharging the brake hydraulic pressure of the wheel brake to a low pressure accumulator (LPA) during the regenerative brake cooperation control and an ESC pump supplying the brake hydraulic pressure of the LPA to the wheel brake as much as a decrease amount of the regenerative brake force when the regenerative brake force decreases, and the outlet valve is closed depending on the pedal stroke additionally generated to supply the brake hydraulic pressure to the wheel brake when the regenerative brake force has a maximum value.
The prior art presents a brake control method for a hybrid electric vehicle that stores the brake hydraulic pressure of the wheel brake in the LPA through the outlet valve of the ESC when the regenerative brake force increases in order to perform the regenerative brake cooperation control according to a pedal stroke signal, supplies the brake hydraulic pressure of the master cylinder to the wheel brake by closing the outlet valve according to the pedal stroke signal additionally generated when the regenerative brake force has the maximum value, and drives the ESC pump to supply the brake hydraulic pressure of the LPA to the wheel brake as much as the decrease amount of the regenerative brake force when the regenerative brake force decreases.
Herein, the pedal stroke signal is a signal output from a brake pedal stroke sensor and in the prior art, the pedal stroke signal depending on a brake pedal pressing degree of a driver is generated by adding the brake pedal stroke sensor and an ESC controller (ECU: electronic control unit) determines a brake will of the driver based on the pedal stroke signal.
FIG. 1 and FIG. 2 are diagrams illustrating a control mode for each brake interval presented in the prior art and as illustrated in FIG. 1 and FIG. 2, the control mode for each brake interval (brake force control mode) includes a first interval {circle around (1)} of performing braking only by the regenerative brake force, a second interval {circle around (2)} of generating the brake hydraulic pressure to satisfy an insufficient part with the hydraulic brake force (the brake force of the hydraulic brake, that is, the friction brake force of the wheel brake that operates by the hydraulic pressure in order to constantly maintain the regenerative brake force and satisfy the total brake force (driver's required brake force), and a third interval {circle around (3)} of increasing the brake hydraulic pressure and the hydraulic brake force as much as the decrease amount of the regenerative brake force when the regenerative braking is released in order to satisfy the total brake force (driver's required brake force).
The first interval {circle around (1)} is an interval in which the driver's required brake force (alternatively, a driver's required brake torque amount) depending on a time is a deceleration of 0.1 g or less and in this interval, braking is available only by the regenerative brake force of the motor without the hydraulic brake force.
In the first interval {circle around (1)}, the brake hydraulic pressure supplied from the master cylinder is stored in the LPA by opening the outlet valve of the ESC to prevent the brake hydraulic pressure of the wheel brake from being increased.
In the second interval {circle around (2)}, the regenerative brake force is maintained at the maximum value (maximum regenerative brake force) formed through the increase of the regenerative brake force in the first interval and when the driver additionally presses the brake pedal, and as a result, when brake force larger than the regenerative brake force is required, the hydraulic brake force (hydraulic brake force) is appropriately increased and decreased to satisfy the driver's required brake force by generating the brake hydraulic pressure.
The second interval {circle around (2)} is an interval in which brake force larger than brake force which may be generated by the regenerative brake force is required and the brake hydraulic pressure of the wheel brake may be increased by receiving the brake hydraulic pressure of the master cylinder formed depending on the pedal operation amount of the driver by closing the outlet valve connected to the LPA.
The third interval {circle around (3)} is an interval in which the regenerative brake is released, and as a result, the regenerative brake amount decreases and in this interval, the hydraulic brake force is increased as large as the decrease amount of the regenerative brake force in order to form the driver's required brake force and to this end, the brake hydraulic pressure stored in the LPA is supplied to the wheel brake by driving the ESC pump to increase the hydraulic pressure of the wheel brake.
However, in the brake control method, when the second interval {circle around (2)} is entered after passing through the first interval {circle around (1)} and thereafter, the first interval {circle around (1)} is re-entered among the brake force control intervals of FIGS. 1 and 2, a problem occurs due to a hydraulic hysteresis phenomenon depending on the brake pedal stroke.
A signal value of the brake pedal stroke sensor, that is, the pedal stroke varies depending on a degree with which the driver presses the brake pedal and the total brake force (driver's required brake force) required by the driver may be acquired as a value which is proportional to the pedal stroke.
For example, the ESC controller (ESC ECU) may calculate the driver's required brake force (may be acquired as a pressure value in a map, bar) from a signal (pedal stroke, mm) of the brake pedal stroke sensor by using a map (pedal stroke Vs pressure) illustrated in FIG. 3.
Referring to FIG. 3, the brake force of 0 bar needs to be generated in the pedal stroke of 2 mm and the brake force of 10 bar needs to be generated in the pedal stroke of 15 mm and the brake force corresponds to the first interval {circle around (1)} of FIG. 1 and in the first interval {circle around (1)}, the regenerative brake torque corresponding to the driver's required brake force is requested to a hybrid control unit (hereinafter, referred to as ‘HCU) to generate the regenerative brake force and the driver's required brake force is satisfied with only the regenerative brake force.
In this case, the regenerative brake torque requested to the HCU may be calculated as below.Regenerative brake torque=driver's required brake force×conversion coefficient.
Herein, the conversion coefficient is a conversion coefficient for converting, when the driver's required brake force is acquired from the map as a hydraulic pressure value, the hydraulic pressure value into a torque value and in the first interval, the hydraulic brake force is not generated, but the hydraulic pressure corresponds to wheel brake pressure (brake hydraulic pressure of the wheel brake) in the second interval and a hydraulic pressure value corresponding to the regenerative brake force (driver's required brake force) is previously defined as a value depending on the pedal stroke in a map diagram.
When the ‘pedal stroke Vs pressure’ map illustrated in FIG. 3 is applied, the pedal stroke to determine the driver's required brake force is a value detected by the brake pedal stroke sensor.
The map of FIG. 3 is a map for determining the driver's required brake force in the first interval and in the first interval, the driver's required brake force is satisfied with only the regenerative brake force without generation of the hydraulic brake force.
Therefore, the driver's required brake force in the first interval may be referred to as the regenerative brake force, but as illustrated in FIG. 3, the map diagram in which the brake hydraulic pressure value of the wheel brake corresponding to the regenerative brake force (that is, driver's required brake force) may be acquired from the current pedal stroke, that is, a map diagram in which the brake hydraulic pressure value is defined as a value depending on the pedal stroke may be used.
In the illustrated map diagram of FIG. 3, a pedal stroke value to enter the second interval from the first interval may be set to 15 mm and in the specification, the pedal stroke value to enter the second interval from the first interval will be referred to as an upper threshold stroke and the driver's required brake force (hydraulic pressure value) in the upper threshold stroke will be referred to as a maximum regenerative brake force pressure corresponding value.
In the map diagram of FIG. 3, when the driver's required brake force decreases due to the decrease in pedal stroke and thereafter, the pedal stroke reaches a predetermined low pedal stroke value, the driver's required brake force is ‘0’ and in the specification, the pedal stroke value in which the driver's required brake force is ‘0’, that is, a pedal stroke value in which the brake force is not generated (regenerative brake force=0) will be referred to as a lower threshold stroke.
In the map diagram of FIG. 3, the lower threshold stroke value is set to 2 mm and when the pedal stroke decreases while braking to reach the lower threshold stroke value, the regenerative brake force is ‘0’ by controlling the regenerative brake torque to ‘0’.
The map diagram of FIG. 3 becomes a linear map diagram in which the driver's required brake force (pressure value) in the lower threshold stroke value is 0 bar and the driver's required brake force in an upper threshold stroke value is the maximum regenerative brake force pressure corresponding value.
In the map diagram of FIG. 3, the upper threshold stroke value is set to 15 mm and a maximum regenerative brake force pressure corresponding value is set to 10 bar.
Therefore, when the driver additionally presses the brake pedal during the regenerative brake cooperation control using the map diagram of FIG. 3, and as a result, the pedal stroke is more than 15 mm which is the upper threshold stroke value, the second interval {circle around (2)} is entered and in the second interval {circle around (2)}, the brake hydraulic pressure is increased to satisfy the driver's required brake force while maintaining the regenerative brake torque, thereby increasing the hydraulic brake force.
In this case, only the hydraulic brake force is appropriately increased and decreased while maintaining the regenerative brake force in order to satisfy the driver's required brake force that varies depending on the pedal stroke which is the driver's pedal press degree.
In the second interval {circle around (2)}, the ESC pump is not also driven while not controlling all of an inlet valve which is a normal open (NO) valve, an outlet valve which is a normal close (NC) valve, and a traction control (TC) valve which is the normal open (NO) valve (the inlet valve is opened, the outlet valve is closed, and the TC valve is opened).
In the second interval {circle around (2)}, pressure generation depending on the pedal stroke has a hysteresis characteristic illustrated in FIG. 4 and FIG. 5.
That is, when required wheel brake pressure increases when the pedal stroke increases to a value more than 15 mm and thereafter, the pedal stroke decreases again to reach 15 mm, the wheel brake pressure is 10 bar in an example of FIG. 4 and the wheel brake pressure is 0 bar when the pedal stroke reaches 20 mm in the example of FIG. 5.
That is, when the pedal stroke decreases to 15 mm or less, the first interval {circle around (1)} is entered and the brake force needs to be generated with only the regenerative brake torque, but in the example of FIG. 4, the regenerative brake torque by the ‘pedal stroke Vs pressure’ map illustrated in FIG. 3 and the hydraulic brake force by the hysteresis phenomenon are simultaneously generated, and as a result, a superimposition phenomenon of the brake force occurs and in an example of FIG. 5, since the regenerative brake torque is not changed while the pedal stroke reaches 15 mm from 20 mm, the brake force is not changed (the wheel brake pressure is 0 bar).
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.